There are a number of factors that may play a role in the development of depression, including genetic, environmental, emotional, psychological and biological influences. According to one integrative neuroscientist, depression could even be a form of infectious disease.
Some research suggests that inflammation can play a large role in the development of depression. This research has tended to focus on inflammatory markers in the blood, and on depression co-occurring with physical ailments that cause an immune reaction.
But according to new research, brain inflammation that occurs independently of physical illness may be highly correlated with clinical depression.
Depression patients display a 30 percent increase in certain markers of brain inflammation when compared with a control group without depression, according to a study from the Center for Addiction and Mental Health (CAMH) at the University of Toronto.
"This finding provides the most compelling evidence to date of brain inflammation during a major depressive episode," the study's senior author Dr. Jeffrey Meyer said in a statement. "This is the first definitive evidence found in the brain."
The research team conducted brain scans on 20 study participants who had depression (but were otherwise healthy), and 20 healthy control participants. Using positron emission tomography (PET), they were able to measure the activation of immune cells (micoglia) that play a key role in activating the brain's inflammatory response.
The researchers found significantly elevated levels of brain inflammation among those with depression, with the highest rates of inflammation occurring among those with the most severe depression.
"Depression is a complex illness and we know that it takes more than one biological change to tip someone into an episode," says Dr. Meyer. "But we now believe that inflammation in the brain is one of these changes and that's an important step forward."
They explained that while inflammation is a response that the brain uses to protect itself, too much inflammation can cause damage, leading to symptoms like poor mood and inability to sleep.
Previous research has linked inflammation resulting from physical health conditions such as cancer and autoimmune disease to the development of depressive symptoms, but the new research is the first to show that inflammation can play a role in depression -- even in the absence of a physical illness.
Current treatments for depression do not target inflammation. Separately, an estimated 10 percent of those on antidepressants do not respond to the treatment, and 20 to 30 percent do not respond to the first antidepressant treatment.
"This discovery has important implications for developing new treatments for a significant group of people who suffer from depression," says Dr. Meyer, who also holds a Canada Research Chair in the neurochemistry of major depression. "It provides a potential new target to either reverse the brain inflammation or shift to a more positive repair role, with the idea that it would alleviate symptoms."